The problem of separating particles of high density such as gold, iron or tin from tailings and other slurry streams has attracted a great many attempted solutions. The problem is that of separating small particles of higher density from a slurry containing water and particles of lower density. One approach has been to use the centrifugal force created in a rotating bowl to separate the high density particles from the lower density slurry. In the past this had been generally done by placing obstructions such as ribs in the path of the rotating slurry to trap the heavier particles. This method had two problems. Where the slurry contained fine, dense particles such as magnetite, the grooves or depressions designed to retain the concentrate would rapidly pack with the unwanted fine particles. Secondly, this was a batch process in that it was necessary to periodically stop the centrifuge to empty it of the concentrate which had been collected.
The problem of packing has been largely solved by the present inventor's batch centrifugal concentrator which is the subject of U.S. Pat. No. 4,824,431. In that centrifugal concentrator, there are no obstacles to the flow of the slurry in the rotating drum. The slurry is delivered to the vicinity of the bottom of the rotating drum and travels up the smooth interior surface of the drum. The interior surface has three continuous zones: an outwardly inclined migration zone, a generally vertical retention zone above the migration zone, and an inwardly-inclined lip zone above the retention zone. The respective lengths and inclinations of the zones are selected to produce flow conditions in which less dense particles are expelled from the drum while denser particles migrate to and are retained in the retention zone. The result is that an enriched layer of concentrate accumulates in the retention zone without the use of ridges or grooves which may become packed.
It remains that this inventor's above-described centrifugal concentrator is a batch device and it is necessary to periodically stop the machine to empty it. In some situations, this periodic stoppage can add to the cost of running the centrifuge. Furthermore, to permit a continuous stream of tailings to be centrifuged would require multiple batch machines and complicated logistics. Also the concentrate retention capacity of the batch type is quite limited. Where the retention zone is flushed frequently the grade of concentrate is low, since a large proportion of non-enriched material is obtained with each flushing of the zone.
Centrifugal concentrators are known for continuous separation of suspended solids from a liquid. U.S. Pat. No. 3,797,736 Gunnewig issued Mar. 19, 1974 discloses a nozzle centrifuge for separating very fine particles from a liquid, having a series of principal discharge nozzles for continuous discharge of concentrate which are dis-posed a shorter distance from the axis of rotation than the outer periphery of the separating chamber. Auxiliary discharge nozzles are provided at the outer periphery of the separating chamber which are valved to periodically open if additional concentrate requires discharge. U.S. Pat. No. 4,432,748 Novoselac et al. issued Feb. 21, 1984 discloses a centrifuge for continuously dewatering a slurry and discharging the solid concentrate. These devices are directed only at removing solids from a slurry, rather than separating solids of a higher specific gravity from those of a lower specific gravity in a slurry. The Novoselac device requires adjustment of the entire bowl configuration to adapt to small variations in the feed parameters and so would have difficulty in practice providing a constant stream of discharge.
There is therefore a need for a continuous discharge centrifugal concentrator which has the advantages of the present inventor's non-packing smooth-flowing batch centrifuge.